This invention relates generally to ink printers and, more particularly, to printers in which an aqueous ink is applied to a porous recording medium such as paper.
Liquid ink printers of the type frequently referred to as continuous stream or as drop-on-demand, such as piezoelectric, acoustic, phase change wax-based or thermal, have at least one printhead from which droplets of ink are directed towards a recording medium Within the printhead, the ink is contained in a plurality of channels. Power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the end of the channels.
In a thermal ink-jet printer, the power pulse is usually produced by a heater transducer or a resistor, typically associated with one of the channels. Each resistor is individually addressable to heat and vaporize ink in the channels. As voltage is applied across a selected resistor, a vapor bubble grows in the associated channel and initially bulges from the channel orifice followed by collapse of the bubble. The ink within the channel then retracts and separates from the bulging ink thereby forming a droplet moving in a direction away from the channel orifice and towards the recording medium whereupon hitting the recording medium a dot or spot of ink is deposited. The channel is then refilled by capillary action, which, in turn, draws ink from a supply container of liquid ink.
The ink jet printhead may be incorporated into either a carriage type printer, a partial width array type printer, or a page-width type printer. The carriage type printer typically has a relatively small printhead containing the ink channels and nozzles. The printhead can be sealingly attached to a disposable ink supply cartridge. The combined printhead and cartridge assembly is attached to a carriage which is reciprocated to print one swath of information (equal to the length of a column of nozzles), at a time, on a stationary recording medium, such as paper or a transparency. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath or a portion thereof, so that the next printed swath is contiguous or overlapping therewith This procedure is repeated until the entire page is printed. In contrast, the page width printer includes a stationary printhead having a length sufficient to print across the width or length of a sheet of recording medium at a time. The recording medium is continually moved past the page width printhead in a direction substantially normal to the printhead length and at a constant or varying speed during the printing process. Partial width array printers are disclosed, for example, in U.S. Pat. No. Re. 32,572 and U.S. Pat. No. 4,638,337. A page width ink-jet printer is described, for instance, in U.S. Pat. No. 5,192,959. These patents are hereby incorporated by reference.
Many liquid inks and particularly those used in thermal ink jet printing, include a colorant or dye and a liquid which is typically an aqueous liquid vehicle, such as water, and/or a low vapor pressure solvent. The ink is deposited on the substrate to form an image in the form of text and/or graphics. Once deposited, the liquid component is removed from the ink and the paper to fix the colorant to the substrate by either natural air drying or by active drying. In natural air drying, the liquid component of the ink deposited on the substrate is allowed to evaporate and to penetrate into the substrate naturally without mechanical assistance. In active drying, the recording medium is exposed to heat energy of various types which can include infrared heating, conductive heating and heating by microwave energy.
Active drying of the image can occur either during the imaging process or after the image has been made on the recording medium. In addition, the recording medium can be preheated before an image has been made to precondition the recording medium in preparation for the deposition of ink. Preconditioning of the recording medium typically prepares the recording medium for receiving ink by driving out excess moisture which can be present in a recording medium such as paper. Not only does this preconditioning step reduce the amount of time necessary to dry the ink once deposited on the recording medium, but this step also improves image quality by reducing paper cockle and curl which can result from too much moisture remaining in the recording medium.
Various drying mechanisms for drying images deposited on recording mediums are illustrated and described in the following disclosures which may be relevant to certain aspects of the present invention.
U.S. Pat. No. 5,005,025, to Miyakawa et al., describes an ink jet recording apparatus for recording which fixes ink through evaporation of an ink solvent. The apparatus includes a heating member extending both upstream and downstream with respect to a recording area and a conveying direction of the recording sheet. The heating member contacts the recording sheet to assist in the fixation of the ink.
U.S. Pat. No. 5,406,321, to Schwiebert et al., describes an ink jet printer and a paper preconditioning preheater therefore. The paper preconditioning preheater has a curved surface and a multi-purpose paper path component to accomplish direction reversal for the paper. The paper contacts the preheater which dries and shrinks the paper to condition it for a printing operation. The preheater is a thin flexible film carrying heater elements which is suspended in air to provide extremely low thermal mass and eliminate the need for long warm up times.
U.S. Pat. No. 5,296,873 to Russell et al. discloses a paper preconditioning preheater in the form of a preheated drive roller which engages the medium and drives it into the print zone.
Copending application U.S. Ser. No. 08/523,322 assigned to the same assignee of the present invention, discloses a segmented heater which includes a curved preheater segment and a planer segment positioned in the print zone. All of the above-identified references are hereby incorporated by reference.
A continuing problem with printer designs which include a recording medium preheating function is the unevenness of the medium warm up as it moves from a medium supply station into the ink print zone. Referring to the prior art design shown in FIG. 1, a recording medium 10 is moved from a supply tray 12 by feed roller 14. Medium 10 is conveyed across the top surface of a heater 16 which is powered by a power supply 18 creating a current in the heater, which may be, but is not limited to, a foil or other type of heating mechanism, and increasing the temperature thereof The medium is moved into a print zone 20, where an ink jet cartridge 22 comprising a printhead 24, connected to an ink reservoir 26, is pulsed by input signals from drive circuit 28 to cause ink droplet ejection in an image-wise pattern on the medium. The medium is advanced into the print zone by drive roller 30 cooperating with a pinch roller 32. The cartridge is moved by a carriage (not shown) back and forth (into and out of the page) and the medium is incrementally advanced following formation of each image line.
Medium 10 is typically advanced quickly along the path extending from the lip of the supply tray (point A) to the nip formed by roller pairs 34, 36 (point B). This initial rapid advance of the medium is done to minimize interprint time and reduce the first print out time. Once the leading edge of the medium enters the print zone, the medium moves through the print zone at the normal medium indexing time (the medium is held stationary until a line is recorded and then indexed forward). It will be appreciated that the first leading portion of the medium (distance from A to B) will be warmed less than the trailing portion of the medium (distance A to the trailing edge). This unequal heating of the paper can lead to differential print quality problems for the first portion of each recorded medium.